By definition, light is electromagnetic radiation that is capable of inducing a visual sensation in the eye. Under this definition, it is common to describe light as being sinusoidal in nature, and as having wavelengths in a range between about four hundred and eight hundred nanometers (400 -800 nm). When light travels as a sinusoidal wave, it will travel through a distance of one wavelength during each of its cycles. Mathematically, these cycles can be expressed as angles where one complete cycle for the sinusoidal wave corresponds to 2.pi. radians or 360.degree..
In order to compare different beams of radiation to each other, when both beams have the same wavelength, it is sometimes helpful to identify the phase difference or phase shift between the beams. For this purpose, phase can be defined as the fraction of a periodic waveform that has been completed at a specific reference time. Thus, if a first ray of light and a second ray of light have the same wavelength, .lambda., and they travel from the same start point through the same medium, but they start at different times, the difference in their respective start times can be expressed as a phase shift. The same argument, of course, applies for their arrival at a common distance from the start point. Thus, for example, if the second ray of light starts one half of a wavelength behind the first ray of light, the phase shift between the first ray of light and the second ray of light will be .pi. radians, or 180.degree.. Due to this phase shift, the first ray of light will arrive at a common destination one half of a wavelength ahead of the second ray of light.
With the above in mind, it is to be appreciated that a beam of light can be considered as being composed of many individual rays of light. Further, though these rays of light may have the same wavelength, at a particular point in space each light ray will have its own characteristic phase that may, or may not, be the same as the phases of the other light rays. Building on this notion, it is to be appreciated that as a beam of light passes at angles through different media, individual rays in the beam will be refracted through different distances. The consequence then is that the different rays of light in the beam will be affected differently. Stated differently, as individual rays of light in the beam travel along paths of different lengths, there will be a phase shift between the rays. This happens, for example, as a beam of light passes through the optical components of an eye (e.g. lens, cornea). One technique for determining the extent of these differences, and their consequences for diagnostic purposes, is known as wavefront analysis.
In one aspect of a wavefront analysis, the individual rays in a light beam are evaluated for their respective phase. Specifically, this is be done as they pass through a common plane. In this case, if all rays in the beam that have the same wavelength, also have exactly the same phase as they pass through this common plane, a plane wavefront results. On the other hand, if the rays in the beam have different phases, the wavefront will be distorted. Depending on the extent, position and nature of the distortions in the wavefront, the optical characteristics of the path traveled by individual light rays in the beam can be analyzed.
With the above in mind, consider an eye. If light can be radiated from a point source on the retina of the eye, this light will initially radiate as a light beam having a plane wavefront. As the beam travels outwardly through the eye, however, it will pass through optical components of the eye which may, or may not, introduce aberrations into the light beam. In turn, these aberrations will manifest themselves as distortions of the wavefront.
Consequently, by determining the wavefront of a light beam, after it has passed through an eye, it is possible to evaluate the wavefront and to determine therefrom whether the aberrations that have been introduced are normal or require corrective action.
In light of the above, it is an object of the present invention to provide a device for imaging a wavefront for evaluating the optical properties of an eye. Another object of the present invention is to provide a device which generates wavefront data based on the phase shifts in a light beam that are caused when light passes through an eye. It is another object of the present invention to provide a device for imaging a wavefront which effectively establishes a point source of light on the retina of the eye. Still another object of the present invention is to provide a device for imaging a wavefront to evaluate the optical properties of an eye that is easy to use, relatively simple to manufacture, and comparatively cost effective.